This invention is directed to improved neoprene latex compositions particularly suitable in contact adhesives.
Neoprene-based adhesives are well known to the art. Most of the earlier adhesives were of the rubber cement type, the base polymer and other additives, such as tackifiers, stabilizers, antioxidants, etc., being dissolved or dispersed in an organic solvent. However, it is expected that the demand for rubber cements will decrease with time because of their flammability, potential health hazard to persons exposed to solvent vapors, and high cost of special installations required to reduce environmental pollution hazards. Water-based neoprene adhesives have been known for many years but a new type has recently become available. The base polymer in these new compositions is a copolymer of chloroprene with an .alpha., .beta.-unsaturated carboxylic acid, such as methacrylic or acrylic acid, and is advantageously prepared in an aqueous emulsion in the presence of a small amount of polyvinyl alcohol. Polymer latices of this type, described in British Pat. No. 1,469,993 (to E. I. du Pont de Nemours & Company), are said to be particularly suitable in adhesives for laminating metals. Normally zinc oxide or magnesium oxide is used in those adhesive formulations as acid acceptor to assure good bond stability.
The same type of latex is used in pressure-sensitive adhesive coatings, as taught by U.S. Pat. No. 3,920,600 to L. Ahramjian. The only difference between the polymerization process of the British Patent and that of the U.S. Patent is that the former can be carried out in the presence of either an alkyl mercaptan or a dialkyl xanthogen disulfide as chain transfer agent, while the latter requires the use of a dialkyl xanthogen disulfide.
Latex adhesives of the type described in British Pat. No. 1,469,993 have also found application in contact adhesives. A typical contact adhesive is applied to both surfaces to be joined and allowed to dry. The two surfaces are then brought in contact with each other, usually under slight or moderate pressure, to effect bonding. The presence of zinc oxide in water-based contact cements is detrimental to the "contactability," especially after drying, while beneficial to bond stability and to hot bond strength.
A method of incorporating up to about 3 parts of ZnO per 100 parts of polymer without significant loss of contractibility on aging involves cooking carboxylated neoprene latex with a resin and ZnO under strictly defined conditions, Discl. No. 14619, Res. Discl. Journal, June 1976, p. 29. Since, however, this method requires an investment in cooking equipment and consumes a large amount of energy, it has not been adopted by the majority of adhesive manufacturers.
Technical bulletin SD-233 of E. I. du Pont de Nemours & Co. describes carboxylated neoprene latices: Neoprene Latex 101 and Neoprene Latex 102. This bulletin discusses on page 9 the chemical stability of these latices, including stability to alkaline compounding ingredients. To attain maximum heat resistance of these latices, the technical bulletin suggests on page 14 to raise their pH to at least 10.
While addition of alkalies to adhesive compositions based on these latices and containing zinc oxide also provides some improvement of contactability, especially after aging, this improvement is of practical magnitude only for those compositions based on low-gel neoprene latices, say, below 15% gel. Medium and high gel latices are, however, frequently used in adhesive compositions and for those compositions sufficient improvement of contactability is not achieved by adding an alkali to such a composition or to the latex itself, prior to blending with other ingredients. Furthermore, contact adhesives based on those latices do not bond at elevated temperatures, for example, above 100.degree. C.
There is, therefore, a great need in the contact adhesive art for a water-based neoprene adhesive having good "contactability" and bond stability in the presence of zinc compounds and able to function within a broad temperature to about 125.degree. C., irrespective of the amount of gel in the composition.